1. Field of the Invention
This invention relates to dither arrangements and in particular, but not exclusively, to dither arrangements for ring laser gyroscopes.
2. Discussion of Prior Art
High accuracy laser gyroscopes require a means to overcome the "lock-in" phenomenon which is a characteristic exhibited by all laser gyroscopes. A common method of providing this means is to apply a mechanical angular oscillation to the laser gyro about its sensing axis. A dither spring may be used to produce this oscillatory motion and to support the laser gyro.
The purpose of conventional dither springs is to oscillate the gyro with respect to a fixed inertial space frame. However the angular motion of the gyro relative to its reaction inertia (i.e. its mounting base) is inversely proportional to the inertias of the gyro and its mounting base, i.e. if these inertias were equal, the gyro and mounting base would oscillate at equal amplitude, and if the mounting base had infinite inertia then all of the dither energy would be used to oscillate the gyro.
In practice, the mounting base does not have infinite inertia because every effort is made to reduce the weight and volume of the Inertial Measuring Unit (IMU) which incorporates the laser gyro(s) and this effectively limits the reaction inertia of the laser gyro mounting base. In an IMU which uses single axis laser gyros, the reaction inertia for each laser gyro comprises a casting, three accelerometers and the other two laser gyros, but in an IMU which uses a tri-axial laser three laser gyros are integrated into a single block. A single dither spring is used to oscillate the block, thereby providing oscillatory angular motion to the three laser gyros simultaneously. The reaction inertia in this arrangement is considerably less because it consists of only three accelerometers and their mount. This results in most of the dither motion being applied to the reaction inertia instead of the tri-axial laser gyro.